Louisiana Gov. Bobby Jindal recently chided the federal government for wasting money monitoring volcanoes, implying that eruptions in the U.S. are both unlikely and impossible to predict. Is that true?

Is a volcano in the U.S. going to blow?Yes. The only question is when. Some geologists are worried that it might be sooner rather than later, given the ominous rumblings recently detected in Yellowstone National Park in Wyoming. Filled with geysers, fumaroles, and other geological hot spots, much of Yellowstone sits atop a giant caldera—which is a collapsed volcano. Volcanoes are openings in the Earth’s crust through which molten rock, ash, and gas periodically escape. Frequently, magma—molten rock—moves beneath the surface, prior to spewing out, and such movement registers as seismic tremors. In 2008 and early this year, geologists recorded an unusually high number of tremors in Yellowstone: 813 in 11 days, the second most intense cluster since recordkeeping began, in 1973. Such heightened activity might signal that an eruption is brewing.

When might that happen?It could happen next week—or possibly 50,000 years from now. If that sounds vague, such is the nature of geologic time. Yellowstone’s last major eruption was about 640,000 years ago. It’s due for another one—overdue, in fact—because scientists have determined that the Yellowstone volcano has erupted on a cycle that’s roughly 600,000 years long. Eventually, all that magma building up below the surface is going to need a place to go. The same is true of other monster volcanoes around the globe. “Although very rare, these events are inevitable,” says University of Bristol geologist Stephen Sparks, “and at some point in the future humans will be faced with dealing with and surviving a super eruption.”

What would a Yellowstone eruption be like?It would be a cataclysm affecting everyone in the entire U.S., and in fact, the entire world. Scientists say the eruption 640,000 years ago blew a hole in the Earth’s crust as big as Connecticut, buried areas as far as 100 miles away in molten lava, and threw up ash that came down as far away as Iowa and Louisiana. Such an event today—even with warning—could be a horror of unimaginable proportions. There would be hundreds of thousands of immediate casualties, and so much ash and dust would be thrown into the atmosphere that it would blot out much of the sun, plunging Earth into a darkened, continuous winter lasting years. Crops would die, and global starvation—and warfare over remaining food stockpiles—might follow. Some experts even say an eruption could be a “species-ending event”—that species being humans.

How many volcanoes are there in the U.S.?After Indonesia and Japan, the U.S. is the most volcano-rich nation on Earth. Currently, the U.S. Geological Survey lists 169 geologically active volcanoes in the U.S. and its territories, most of them in Alaska, Hawaii, the Cascade Range of the Pacific Northwest, and California. Of that number, 65 are “historically active”—that is, there is written documentation of past eruptions.

How dangerous are they?It’s hard to say. The Geological Survey employs a five-category scale that ranges from “very low threat” to “very high threat.” On that basis, 18 U.S. volcanoes are considered a “very high threat” to public safety, with another 36 classified as a “high threat.” But geologists can only guess at their destructive power at any given time. Before the May 18, 1980, eruption of Mount St. Helens, a number of small earthquakes prompted authorities to declare a state of emergency and order evacuations. But no one could have known that Mount St. Helens would be the deadliest U.S. volcano since 1915. Exploding with the force of 1,600 Hiroshima-type atomic bombs, it killed 57 people and caused $2.7 billion in damage. “We’re pretty good at the when,” says John Ewert, a volcanologist in Washington state, “but not so good at the what or how big.”

How are volcanoes monitored?Geologists rely mainly on seismometers that continuously send information, via radio, to a central recording site for analysis. They also watch the ground for visible signs of swelling or cracking. Among their instruments are lasers to measure shifts in distance between established benchmarks, and electronic “tiltmeters” that can determine tiny changes in slope angles. But the system is far from foolproof. An internal Geological Survey report in 2005 found that only a few of the nation’s volcanoes are “well monitored,” and that there are no real-time, ground-based monitors at 13 “very-high-threat” volcanoes and at 19 “high-” or “moderate-threat” ones.

Why isn’t more being done? It’s a matter of money. When Gov. Jindal criticized President Obama’s stimulus bill last month, he cited “$140 million for something called ‘volcano monitoring’” as an example of wasteful spending. But Jindal—whose flat, low-lying state has no volcanoes—actually overstated the figure. The $140 million is the amount earmarked for all projects conducted by the Geological Survey. Of that, volcano monitoring accounts for about $15 million. For those in harm’s way, it is a small price to pay. “Does [Jindal] have a volcano in his backyard?” asks Royce Pollard, the mayor of Vancouver, Wash., which is about 70 miles from Mount St. Helens. “We have one that’s very active, and it still rumbles and spits and coughs. We lost lives the last time, and we could lose them again.”

The last big oneThe most devastating volcanic eruption of the last 150 years took place on Aug. 26, 1883, on the island of Krakatoa in the Dutch East Indies. After a series of rumblings that began in May, the volcano exploded with the power of 13,000 atomic bombs, making the loudest sound in human history—it was heard 3,000 miles away. As it ejected a column of ash 17 miles high into the sky, the 2,600-foot volcanic cone collapsed into its center, destroying most of the island and triggering a colossal 130-foot-high tsunami. An estimated 165 villages were wiped out, killing 36,000 people. Ash from the explosion blocked out so much of the sun’s heat that global temperatures soon dropped by about two degrees Fahrenheit; in much of the world, 1884 was called “the year without summer.” What’s left of Krakatoa is located on a major fault line, and a new volcano is now building near the site of the old one. “Sooner or later,” says Australian geologist Richard Arculus, “it will happen again.”